Radio frequency (RF) transmitters, such as used in cellular telephones, develop an RF signal to be transmitted through the air. Information is carried in the signal via some form of modulation such as frequency modulation, phase modulation, amplitude modulation, or a combination of these.
It may be desirable to create a modulated signal with both amplitude and phase modulation. With the necessity of developing small and lightweight devices, particularly cellular telephones, it is important that such amplifier circuits use a minimum of components. One possibility is to use a combination of phase modulation of a master oscillator at a carrier frequency and amplitude modulation of a power amplifier. However, unintended phase modulation can occur when the power amplifier is amplitude modulated. This problem has been addressed by including the amplitude and modulated power amplifier within the total phase locked loop. This can cause the out of band signal that occurs if as a loop is locking to be broadcast unless a switch with very large isolation is included. The feedback point from the voltage-controlled oscillator can be shifted to the output of the power amplifier only after the loop has locked and is on frequency. This shift can cause disruptions of the loop and cause the loop to lose lock.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.